The life forms in a natural body of water are interlinked through a variety of relationships. In the ideal situation plants and algae are the producers that form the basic food resource for the pond ecosystem. Plankton feed off the plants and algae and in turn are eaten by the next larger organisms. Fish eat these bacteria and plankton or the subsequent links in the food chain. Some fish are pure herbivores. Fish excreta, dead plankton, and plant parts, are further converted by floor-dwelling organisms and ultimately are mineralised by the microorganisms. In the mineralisation process organic compounds are oxidised and nutrients such as nitrate or phosphate are simultaneously released. These nutrients in turn are available to plants and algae for their growth.
Thus a complete circle is closed. This cycle illustrates the extreme interdependence of the individual life forms. This interrelationship is also referred to as the food chain or even better, the food web. The stability of the food chain has crucial significance for the biological balance. The greater the number of different species, the more stable is the food chain, and thus the more stable the entire ecosystem. Thus external intervention in one part of the food chain affects all organisms.
Because energy losses and substance losses of up to 90% occur for each "eat" and "be eaten" iteration the food chain is often presented as a pyramid (Fig. 4). Here is a simple example to illustrate this relationship: 10 kg of meat can be generated with 100 kg of soybean flour, the meat can then be used for human nutrition. If human beings would eat the soybean flour directly, 10 times as many people could subsist on the soybean flour.